Charmed Life by Diana Wynne Jones

After a childhood of uneasy dependence, two orphaned siblings are taken to live in a great, slightly forbidding castle where an enchanter who oversees magic trains the gifted and watches over dangerous powers. The sister, brilliant and selfish with a knack for dramatic, risky spells, uses her charm and talent to manipulate others and push the quieter brother into the background, while the brother—timid, ghostlike and thought to be less magical—slowly discovers an extraordinary inner power and a deeper truth about their past. As secrets, rivalries and the rules of magic begin to unravel, he must find courage, outwit his sister’s schemes, and help restore balance before their dangerous games destroy the lives they both depend on.

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